The present invention relates to a method for manufacturing a sensing element of a gas sensor. The gas sensor is generally installed in an exhaust gas passage of an internal combustion engine for combustion control or emission control of the internal combustion engine.
A gas sensing element is necessary to control the combustion of an internal combustion engine. The gas sensing element has a cup-shaped solid electrolytic body having a reference gas chamber formed therein. An inside electrode is provided on an inner surface of the solid electrolytic body. An outside electrode is provided on an outer surface of the solid electrolytic body. A porous layer is provided to cover the outside electrode.
When the gas sensing element is manufactured, a solid electrolytic body having a predetermined shape is first prepared. The inside electrode and the outside electrode are respectively provided on the inner and outer surfaces of the solid electrolytic body. Next, the solid electrolytic body is dipped into a slurry to coat a slurry film on the outside electrode. Then, the slurry film is sintered to form the porous layer.
The dipping method of the solid electrolytic body includes a step of dipping the solid electrolytic body into a slurry stored in a dipping tank. To perform the dipping treatment of slurry uniformly and effectively, the dipping tank has large size sufficient for dipping a plurality of solid electrolytic bodies into the slurry at a time. An appropriate jig is prepared to hang the plurality of solid electrolytic bodies so as to be positioned above the dipping tank. The jig is then lowered toward the dipping tank to dip the plurality of solid electrolytic bodies into the slurry stored in the dipping tank.
The dipping tank is equipped with a stirrer provided on the bottom of this dipping tank. The stirrer rotates in the slurry to cause a flow of slurry in a circumferential direction.
It is also possible to use an independent dipping tank for separately dipping each solid electrolytic body into a slurry stored in this independent dipping tank. Namely, the number of independent dipping tanks is equal to the number of solid electrolytic bodies to be dipped simultaneously. By lowering the jig hanging the plurality of solid electrolytic bodies into respective dipping tanks, each solid electrolytic body is dipped into the slurry stored in the corresponding dipping tank.
Each solid electrolytic body is rotatable about its own axis when hung down from the jig. When the solid electrolytic body is dipped into the slurry, the solid electrolytic body rotates in the slurry. It is however possible to stop the rotation of the solid electrolytic body when the solid electrolytic body is dipped into the slurry.
It is possible to remove the stirrer when the solid electrolytic body rotates about its own axis when hung down from the jig.
However, according to the conventional dipping method for the solid electrolytic body, the slurry concentration and the slurry component in the dipping tank tends to be nonuniform when a slurry having high viscosity is used. The condition of the slurry film coated on the outside electrode becomes different in each solid electrolytic body.
In other words, performance and characteristics of the porous layer is not constant according to the conventional dipping method. The manufactured gas sensors will have performances different from each other.
In view of the problems of the conventional dipping method, the present invention has an object to provide a method for manufacturing a gas sensing element having a uniform porous layer.
In order to accomplish the above and other related objects, the present invention provides a first method for manufacturing a gas sensing element which has a cup-shaped solid electrolytic body having a reference gas chamber formed therein, an inside electrode provided on an inner surface of the solid electrolytic body, an outside electrode provided on an outer surface of the solid electrolytic body, and a porous layer formed so as to cover the outside electrode. The first manufacturing method comprises a step of forming the solid electrolytic body, a step of providing the inside electrode on a predetermined portion of the inner surface of the solid electrolytic body and providing the outside electrode on a predetermined portion of the outer surface of the solid electrolytic body, and a step of dipping the solid electrolytic body into a slurry which is prepared for forming the porous layer by using a dipping apparatus.
The dipping apparatus used in the first manufacturing method comprises a dipping tank for storing the slurry which is prepared for forming the porous layer, a viscosity adjusting tank equipped with a viscosity sensor for measuring a viscosity of the slurry and a viscosity adjusting mechanism for adjusting the viscosity of the slurry based on a sensing value of the viscosity sensor, a fluid passage for connecting the dipping tank and the viscosity adjusting tank, and a circulating pump provided in the fluid passage for forcibly circulating the slurry between the dipping tank and the viscosity adjusting tank.
The dipping step of the first manufacturing method includes a step of dipping the solid electrolytic body into the slurry stored in the dipping tank while the slurry is forcibly circulated between the dipping tank and the viscosity adjusting tank by the circulating pump, a step of forming a slurry film on a predetermined portion of the solid electrolytic body through this dipping treatment, and a step of sintering the slurry film to form the porous layer.
The dipping apparatus of the first manufacturing method comprises the viscosity adjusting tank. The slurry is forcibly circulated between the viscosity adjusting tank and the dipping tank. Thus, the slurry is always stirred. The concentration and viscosity of the slurry used for dipping solid electrolytic bodies can be always kept to a uniform and constant value.
It becomes possible to accurately equalize the condition of a slurry film coated on each solid electrolytic body. Hence, performance and characteristics of the porous layer are constant in each solid electrolytic body. The manufactured gas sensors show the same performances.
The porous layer of the gas sensing element of this invention functions as trap layer.
The outside electrode needs to be exposed to a measured gas atmosphere during detection of gas concentration. When the measured gas contains poisonous or harmful substances, the trap layer is provided to protect (i.e., cover) the outside electrode and the gas sensing element. The trap layer traps the poisonous or harmful substances and assures accurate detection of gas concentration.
Furthermore, it is possible to provide an additional layer on the porous layer. Furthermore, it is possible to provide a second trap layer on the porous layer.
Furthermore, when the outside electrode is provided on the solid electrolytic body and the additional layer is provided on the outside electrode, it is preferable to provide the porous layer so as to cover the additional layer according to the manufacturing method of the present invention. The additional layer is, for example, a protective layer or the like as shown in the later-described embodiments.
In general, the porous layer of the present invention is provided to cover the entire surface of the outside electrode. However, it is possible to provide the porous layer so as to partly cover the outside electrode.
Furthermore, it is possible to provide the porous layer so as to cover other portion of the solid electrolytic solid electrolytic body other than the outside electrode.
Furthermore, the dipping apparatus used in the first manufacturing method is equipped with the viscosity adjusting tank. The viscosity adjusting tank has a function of maintaining the viscosity (i.e., concentration) of slurry stored in the dipping tank at a constant value. Accordingly, it becomes possible to prepare uniform slurry preferable for the dipping treatment.
The viscosity adjustment of slurry is performed by the viscosity adjusting mechanism. The viscosity adjusting mechanism is responsive to a sensing value of the viscosity sensor. When the viscosity of slurry deviates from a predetermined value, the viscosity adjusting mechanism adds an appropriate amount of water into the viscosity adjusting tank based on a sensing value of the viscosity sensor.
Furthermore, the slurry is always and forcibly circulated. This is effective to prevent the surficial slurry film from appearing on the surface of the slurry. It is also effective to prevent the slurry from sedimenting on the bottom of the dipping tank.
According to the first manufacturing method of the present invention, it is preferable that the dipping tank has a fluid inlet port for introducing the slurry from the viscosity adjusting tank, a receiving groove is provided along an opening of the dipping tank so as to surround a circumferential upper end of the dipping tank to receive the slurry overflowed from the dipping tank, and a fluid outlet port provided in the receiving groove for discharging the overflowed slurry to the viscosity adjusting tank.
With this arrangement, it becomes possible to circulate the slurry in a wide region ranging from the bottom to the upper opening of the dipping tank. Especially, it becomes possible to promote the flow of slurry in the vicinity of the surface of slurry stored in the dipping tank.
According to the first manufacturing method of the present invention, it is preferable that a baffle is provided in the dipping tank so as to be positioned adjacent to the fluid inlet port so that flow of the slurry introduced from the viscosity adjusting tank collides with the baffle.
According to this arrangement, the slurry flowing into the dipping tank from the fluid inlet port is guided by the baffle so as to diffuse in all radial directions (refer to FIG. 4). The flowing speed of the slurry increases. The slurry is smoothly circulated upward from the bottom of the dipping tank. It becomes possible to eliminate the sediment of slurry.
A predetermined gap is provided between the baffle and the fluid inlet port so that the introduced slurry smoothly flows.
According to the first manufacturing method of the present invention, it is preferable that a porous stirring plate is provided in the dipping tank, and the stirring plate oscillate in the dipping tank.
With this arrangement, it becomes possible to surely stir the slurry stored in the dipping tank. It becomes possible to eliminate the sediment of slurry. Furthermore, it becomes possible to provide the slurry having constant and uniform concentration and viscosity.
Furthermore, it is preferable that the stirring plate has a configuration just fitting to the inner wall of the dipping tank (refer to FIG. 6). This realizes easy stirring of slurry stored in the dipping tank.
Furthermore, the present invention provides a second method for manufacturing a gas sensing element which has a cup-shaped solid electrolytic body having a reference gas chamber formed therein, an inside electrode provided on an inner surface of the solid electrolytic body, an outside electrode provided on an outer surface of the solid electrolytic body, and a porous layer formed so as to cover the outside electrode. The second manufacturing method comprises a step of forming the solid electrolytic body, a step of providing the inside electrode on a predetermined portion of the inner surface of the solid electrolytic body and providing the outside electrode on a predetermined portion of the outer surface of the solid electrolytic body, and a step of dipping the solid electrolytic body into a slurry which is prepared for forming the porous layer by using a dipping apparatus.
The dipping apparatus used in the second manufacturing method comprises a dipping tank for storing the slurry which is prepared for forming the porous layer, a first stirrer shiftable in a circumferential direction along an opening of the dipping tank to stir the slurry, and a second stirrer rotatable in the vicinity of a bottom of the dipping tank to stir the slurry.
The dipping step of the second manufacturing method includes a step of dipping the solid electrolytic body into the slurry stored in the dipping tank, a step of forming a slurry film on a predetermined portion of the solid electrolytic body through this dipping treatment, and a step of sintering the slurry film to form the porous layer.
According to the second manufacturing method, the slurry is stirred by the first stirrer shiftable in the circumferential direction along the opening of the dipping tank. Furthermore, the slurry is stirred by the second stirrer rotatable in the vicinity of the bottom of the dipping tank. Hence, the first stirrer and the second stirrer are effectively used to stir the slurry from the bottom to the vicinity of the opening of the dipping tank. Thus, it becomes possible to entirely stir the slurry stored in the dipping tank.
The concentration and viscosity of the slurry used for dipping solid electrolytic bodies can be always kept to a uniform and constant value.
It becomes possible to accurately equalize the condition of a slurry film coated on each solid electrolytic body. Hence, performance and characteristics of the porous layer are constant in each solid electrolytic body. The manufactured gas sensors show the same performances.
The dipping apparatus used in the second manufacturing method is a single tank type which is simple in arrangement. This is effective to manufacture the gas sensing element at low cost.
Furthermore, the slurry can be effectively stirred in the vicinity of the bottom of the dipping tank as well as in the vicinity of the upper opening of the dipping tank. This is effective to prevent the surficial slurry film from appearing on the surface of the slurry. It is also effective to prevent the slurry from sedimenting on the bottom of the dipping tank.
According to the second manufacturing method of the present invention, it is preferable that the first stirrer is a stirring rod provided in the vicinity of the opening of the dipping tank so as to be shiftable in the circumferential direction along an inner wall surface of the dipping tank, and the second stirrer is rotary vanes rotatable about a rotary shaft provided on the bottom of the dipping tank.
With this arrangement, it becomes possible to simultaneously stir the slurry in the vicinity of the bottom of the dipping tank as well as in the vicinity of the upper opening of the dipping tank. Accordingly, it becomes possible to thoroughly stir the slurry stored in the dipping tank. Furthermore, it becomes possible to prevent the surficial slurry film from appearing on the surface of the slurry. Thus, this arrangement is preferable for the slurry having higher viscosity or applicable to the dipping treatment using a large-size dipping tank. When a large-size dipping tank is used, numerous solid electrolytic bodies can be dipped at a time. This improves the productivity.
According to the first or second manufacturing method of the present invention, it is preferable that a ceramic protective layer is formed on the solid electrolytic body before the solid electrolytic body is dipped into the slurry.
According to the first or second manufacturing method of the present invention, it is preferable that the solid electrolytic body is rotated when the solid electrolytic body is dipped into the slurry. This is effective to form a uniform slurry film on each solid electrolytic body.
According to the first or second manufacturing method of the present invention, it is preferable that the solid electrolytic body is stationary when the solid electrolytic body is dipped into the slurry. This arrangement is appropriate to use a large-size dipping tank which is capable of dipping many solid electrolytic bodies at a time. The efficiency of dipping treatment can be improved.
In any of the first and second manufacturing method of the present invention, it is possible to form a uniform and same porous layer on each solid electrolytic body.